The present invention relates to a power semiconductor device and a manufacturing method thereof, and more particularly to a power semiconductor device employing a field plate and a manufacturing method thereof.
As the scale and capacitance of appliances become large, the power semiconductor device must have a high breakdown voltage at high current. Also, to pass high current with low power consumption, the power semiconductor device requires a low saturation voltage. A high breakdown voltage characteristic is further required to withstand a high voltage of a reverse direction applied to the power device in an OFF state, and at the moment that a switch is turned off.
The breakdown voltage of the semiconductor device is determined by the size of a depletion layer of an associated pn junction. This is because most of the voltage applied to the pn junction drops across the depletion layer. It is known that a curvature in the shape of the depletion layer also affects the breakdown voltage. That is due to an electric field crowding effect, whereby an electric field is more concentrated on a portion having a curvature than on a plane portion. The effect is exacerbated at edges of the depletion layer having a high curvature. Accordingly, avalanche breakdown is more easily generated at such edges, which reduces the breakdown voltage of the entire depletion layer.
A method for dealing with this problem in the prior art is disclosed in Power Semiconductor Devices, pp. 100-102, written by B. J. Baliga, 1996. The method is by forming an insulator and a field plate on the edge portion of the depletion layer. A voltage applied to the field plate controls the surface electric potential, which in turn controls the size and shape of the depletion layer. Thus the applied voltage can improve the size and curvature of the depletion layer, and thereby increase the breakdown voltage.
The breakdown voltage of a power semiconductor device is a value obtained by integrating the electric field E in the depletion layer with respect to a horizontal length `X` of the depletion layer. Accordingly, when the field plate voltage is applied such that it increases the length of the integrated area, the breakdown voltage increases. This happens because the electric field concentration in each curved portion of the depletion layer is reduced, which thereby prevents avalanche breakdown at each such portion.